Jon Willis: Research

• When did the first galaxy clusters form?
• How does the cluster environment affect galaxy evolution?
• When did the first galaxies appear?
• What are the properties of dark matter in galaxies and clusters?

• Galaxy Evolution in X-ray Clusters.

• Galaxy clusters are vast collections of galaxies. The galaxies we see are only the tip of a great gravitational iceberg, making up only about 1% of the total mass of any given cluster. About 16% of the mass is in the form of a hot cloud of protons and electrons. At a temperature of some 10 million Kelvin this cloud of ions produces luminous X-ray photons. The rest of the iceberg is dark matter. We can be certain of two things concerning the dark matter: yes, it really is there and no, it doesn't emit any light. To detect galaxy clusters you can perform one of the following observations: 1) take an optical image and look for dense collections of galaxies, 2) take an X-ray image and look for cluster-sized clouds of hot gas, 3) take an optical image and look for evidence of gravitational lensing where the unseen dark matter creates a giant magnifying glass in the sky.
  

• I am a member of the FornaX survey. This is a multi-wavelength survey primarily based upon the X-ray detection of distant galaxy clusters using the XMM satellite and optical and infrared imaging and spectroscopy using the Euclid space mission.

• I continue to be a member of the XXL survey. This XMM program covers 50 square degrees of the sky and constitutes the largest single project attempted with XMM.
  

• XXL has discovered over 500 new galaxy clusters and is supported by a ground-based large observing program at the European Southern Observatory to determine cluster redshifts.

• My own work has focused on the search for distant galaxy clusters in FornaX, XXL and it predecessor, the XMM-LSS survey (see Willis et al. 2013).

• One highlight from this work is the discovery of the galaxy cluster XLSSC 122 at a redshift of z=1.98 (Willis et al. 2020). This cluster appears to be relatively massive and mature yet is observed only 3.4 Gyr after the Big Bang.
  

• My graduate students have performed detailed studies of XLSSC 122 (e.g. Lonsbrough et al. 2026, Leste et al. 2024 and Trudeau et al. 2022), searches for further distant clusters (Trudeau et al. 2020), the properties of the brightest cluster galaxies (BCGs) and how they depend upon the cluster they inhabit (Lavoie et al. 2016), in addition to studies of the variation in the red/blue galaxy mix in X-ray clusters in an attempt to isolate and understand the effect of the cluster environment on galaxy evolution (Urquhart et al. 2010).

• The FornaX survey continues to be a rich source of new projects - whether at the graduate or undergraduate level. Please contact me for more details.
  

• ZEN: Ultra-deep narrow band searches for Lyman alpha emission at redshifts z > 7.

• When did the first galaxies appear? The most distant galaxies currently identified lie at redshifts z ~ 7 - within 1 billion years of the big bang. Detecting galaxies at earlier cosmic times represents a considerable observational challenge. However, the scientific motivation for doing so is very great: the observation of galaxies at z > 7 provides a direct view of the very first galaxies in the act of formation. In addition, complementary observations of galaxies and quasars at z = 6 indicate that at this time the universe underwent a fundamental change - the end of the epoch of reionisation - when the energy input from young stars and AGN converted the baryonic contents of the universe to a largely ionised state. Galaxies at z > 7 exist before this universal change was complete and the line-of-sight they illuminate provides an important probe of the physical state of the universe. In order to detect the first galaxies at z > 7, I initiated the ZEN (z equals nine) project:
  

• ZEN1 consisted of a 32 hour Very Large Telescope (VLT) ISAAC (Infrared Spectrometer And Array Camera) image of the Hubble Deep Field (HDF) South taken in a narrow-band filter (NB119).  By combining this new data with existing, ultra-deep images taken at other wavebands we performed a sensitive test for the presence of star-forming galaxies at z ~ 9. We discovered no such galaxies in this (admittedly small) field - see Willis and Courbin (2005) for details.
  

• ZEN2 is an extension of the ZEN project to look at three massive, lensing clusters with the same combination of narrow and broad band filters used in ZEN1. The foreground lens clusters (A1835, AC114 and A1689) form a magnified view of the high-redshift universe and provide a boost to the light levels from distant star-forming galaxies. No clear ZEN galaxies were detected - see Willis et al. (2008) for details.
  

• ZEN3 used the Canada France Hawaii Telescope's new wide-field infrared camera WIRCam to search for high redshift galaxies. Although CFHT is a 4-metre telescope, compared to the 8m VLT, the efficiency of the telescope plus instrument combination ensures that our 40 hour NB image reached a significant level of the sensitivity of the previous ultra-deep images. The advantage of using WIRCam is the very large field of view: its 20x20 arcminute field of view is approximately 60 times larger than VLT/ISAAC. Therefore, though ZEN3 is a marginally shallower survey than ZEN1 and ZEN2, we accessed a much larger volume of the universe. The survey generated a number of candidate high-redshift galaxies. However, the candidates are of marginal quality and to date none have been confirmed by spectroscopy. See Hibon et al. (2010) for details.
  

• I have continued to obtain further narrow-band observations with the aim of detecting z>7 galaxies, both as part of a large international collaboration using the HAWK-I camera at the VLT and using CFHT/WIRCam to conduct a wide field survey for very bright Lya emitting galaxies.
  

• These surveys have also generated very large samples of faint line emitting (i.e. star forming) galaxies at redshift z<7. These samples are some of the largest and deepest compiled to date.

• Discovering and studying gravitational lenses in the SDSS and CFHLS surveys:

• What are the properties of dark matter in galaxies? Gravitational lensing provides a direct probe of the dark matter distribution in a particular lens system. The difficulty is to find examples of foreground galaxies lensing spatially extended background galaxies. The key point is that extended background galaxies provide an incredibly sensitive probe of both the amount and distribution of dark matter in the foreground galaxy. Finding such systems has proven difficult but if a sample of several tens of such "galaxy-galaxy" lenses can be identified then a detailed dark matter map can be created for the lensing galaxies - a key ingredient in understanding both the physical nature of dark matter and the physics of galaxy formation.
  

• The Optimal Line-of-Sight (OLS)  Lens survey used the Sloan Digital Sky Survey (SDSS) to discover luminous red galaxies (LRGs) lensing background star-forming galaxies. The trick was to look at every LRG spectrum in Sloan to discover the presence of unexpected (background) emission line galaxies. This project discovered 9 new gravitational lenses - see Willis et al. (2005) and Willis et al. (2006) for details.
  

• Searching for new gravitational lenses.  One of my past graduate students - Karun Thanjavur - completed a search for new, bright cluster lenses in the Canada France Hawaii Telescope Legacy Survey (CFHTLS) Megacam images. His aim is to discover lensed star-forming galaxies that are bright enough to perform spatially resolved spectroscopy on using 8-10m class telescopes. His work generated one of the largest catalogues of galaxy clusters drawn from the CFHTLS - see Thanjavur et al. (2009) - in addition to using a combination of gravitational lensing and cluster galaxy dynamics to determine the dark matter mass properties of the two of the clusters identified as lensing background galaxies - see Thanjavur et al. (2010).